1. Field of the Invention
This invention pertains generally to control rod drive mechanism seismic supports for nuclear power plants and more particularly to a quick disconnect seismic support tie rod system.
2. Description of the Related Art
In conventional reactors, the head package includes the pressure vessel head which seals the reactor vessel, control rod drive mechanisms which are used to raise and lower control rods in the core of the reactor, a seismic platform adjacent the upper ends of the control rod drive mechanisms, which laterally restrains the drive mechanisms, and various cables for operation of the control rod drive mechanisms. A missile shield, which conventionally was formed of a concrete slab, is positioned above the head package to protect the containment housing and associated equipment from penetration by any of the control rod drive mechanisms in the event of a major pipe break. The problems associated with such conventional head packages are more fully described in U.S. Pat. No. 4,678,623, issued Jul. 7, 1987, and assigned to the assignee of this invention. In such conventional plants, the large concrete slabs installed above the reactor vessel to act as a missile shield must be removed and stored prior to head disassembly and refueling of the reactor, and then must be replaced after the refueling and head reassembly. Such operations affect overall refueling time and radiation exposure and require space in the containment area for placement of the missile shield slabs when removed from the position above the reactor vessel.
In order to reduce the refueling time, personal exposure and space requirements, an improved system, designated as an integrated head package was developed which incorporates an integral missile shield and head lift rig. The missile shield is in the form of a perforated circular plate which is directly attached to a head lift rig. Such an integral head package system is described in U.S. Pat. No. 4,830,814, issued May 16, 1989 and assigned to the assignee of this invention.
As described therein, and illustrated in FIG. 1 of the present drawings, an integral head package 10 includes a three-legged head lifting rig 12 that is pin connected at 14 by lift lugs 16, to a missile shield assembly 18. The perforated circular plate 20 that forms the missile shield 18 acts as a spreader for the head lift load, and as a seismic support for the tops of the control rod drive mechanisms 22, with rod travel housings extensions 24 of the control rod drive mechanisms protruding through apertures 26 in the circular plate 20. The missile shield 18 interfaces with the tops of the control rod travel housings 22 which limits the overall vertical travel (and impact force) of a missile before it impacts the shield. The impact load of the missile against the underside of the perforated plate 20 is transmitted to head lift rods 28, through vessel head lift lugs 30 secured to the vessel head 32, and closure studs 34 to the vessel head 32, and ultimately to the vessel supports. A cooling shroud 36 surrounds the control rod drive mechanisms 22, while electric cabling 38 is routed from the top of the control rod drive mechanisms 22 to a connector plate 40 and then along a cable tray 42 to respective cable terminations. Cooling fans 44 circulate air within the shroud 36 to transfer waste heat from the control rod drive mechanisms 22. Hoist supports 46, and trolleys 48 on hoist assemblies 50 are used to position stud tensioner tools and stud removal tools during refueling operations.
The integrated head package and variants of the design which have since evolved, were a marked improvement over conventional head package designs, and are adaptable for retrofitting existing reactors or for incorporation into new reactor designs as will be described hereafter. However, there is still room for improvement in reducing the number of steps that have to be performed in the critical path of a refueling outage. For example, many nuclear power plants have control rod drive mechanism seismic support tie rods. Typically, there are five to six tie rods which are pinned at the reactor head assembly attachment, and pinned at a refueling cavity wall mounted anchor. During plant refueling, the tie rods need to be removed in order to move the vessel head assembly to the head storage stand. The ends of the tie rods that are attached to the wall mounted anchors (and in most cases the head assembly mounted ends also) are disconnected and reconnected by operators in a man basket supported by the overhead polar crane. Because these activities utilize the polar crane, which is also required for numerous refueling activities, they are considered to be in the critical path of the refueling outage. Any reduction in critical path time results in significant savings in the form of the refueling schedule and electric utility dollars.
Accordingly, a new tie rod support system is desired that can reduce the number of steps required to disconnect the tie rod wall anchors from the vessel head so that the vessel head can be removed.
Furthermore, a new tie rod support system is desired that can remove the disconnection of the tie rods from the refueling outage critical path.
Additionally, such a system is desired that can enable the vessel head assembly to be removed from the vessel with the tie rods attached to the head assembly.